Modulation of the TGFβ/Smad signaling pathway in mesangial cells by CTGF/CCN2

Transforming growth factor-beta (TGFβ) drives fibrosis in diseases such as diabetic nephropathy (DN). Connective tissue growth factor (CTGF; CCN2) has also been implicated in this, but the molecular mechanism is unknown. We show that CTGF enhances the TGFβ/Smad signaling pathway by transcriptional suppression of Smad 7 following rapid and sustained induction of the transcription factor TIEG-1. Smad 7 is a known antagonist of TGFβ signaling and TIEG-1 is a known repressor of Smad 7 transcription. CTGF enhanced TGFβ-induced phosphorylation and nuclear translocation of Smad 2 and Smad 3 in mesangial cells. Antisense oligonucleotides directed against TIEG-1 prevented CTGF-induced downregulation of Smad 7. CTGF enhanced TGFβ-stimulated transcription of the SBE4-Luc reporter gene and this was markedly reduced by TIEG-1 antisense oligonucleotides. Expression of the TGFβ-responsive genes PAI-1 and Col III over 48 h was maximally stimulated by TGFβ + CTGF compared to TGFβ alone, while CTGF alone had no significant effect. TGFβ-stimulated expression of these genes was markedly reduced by both CTGF and TIEG-1 antisense oligonucleotides, consistent with the endogenous induction of CTGF by TGFβ. We propose that under pathological conditions, where CTGF expression is elevated, CTGF blocks the negative feedback loop provided by Smad 7, allowing continued activation of the TGFβ signaling pathway.